Development and evolution of folds, faults and fractures within a thrust sheet in fold-thrust belt, Guoshing, central Taiwan

Based on tectonic analysis on the well-exposed riverbed outcrops in the Guoshing area, central Taiwan, we elucidate the deformation structure pattern and its evolution during the upward propagation of a thrust sheet in fold-thrust belt. We mainly focused on a section of continuous outcrops about 1 km long in the hanging wall of the Shuangtung Fault, a major thrust fault in the western foothills of the Taiwan mountain belt. By detailed field investigation we established a local geological 3-D architecture of deformed strata, which is characterized by two to three duplex structures accompanied by multiple folds, which plunge exclusively toward the North. We conducted fault slip data and fracture analysis via Faultkin software. By further comparing the brittle fractures with fold structure, our results show that slickenside faulting occurred on multiple phase throughout the propagation of the thrust sheet on the hanging wall of the Shuangtung fault and that the multiple wave folding seemingly developed in the late stage during the exhumation of the thrust sheet. We are finally able to summarize the evolution of the deformation structures in Guoshing area as following: 1) one early syn-sedimentary faulting with NNW-SSE extension, 2) two phases of compressional thrusting events; one is early conjugate thrust system with E-W compression which may indicate the beginning of thrusting when the strata of this thrust sheet still kept horizontal at depth of about 4-6 km. Then, the hanging wall strata have been tilted through upward movement of the thrust sheet along the Shuangtung Fault. During this period, a late conjugate thrust system (with NNE to E-W compression) developed at the shallow depth of 2-4 km. 3) late stage strike-slip faulting, which often is found to cut across the thrust slickenside. We found that the late stage of slickenside faulting events is likely syn-folding with the multiple folds of the wave length of about 60-80 meters. Our observation and analysis also shows that parts of late stage thrust followed the pre-existing fractures developed in the earlier stages. However, whether the strata have experienced block rotation still needs further paleomagnetic study.